The present invention relates to a pad driver and more particularly to an open drain, active low driver.
Computer devices are increasingly using the PCI bus architecture. Integrated circuit designs used with the PCI bus now optionally require to have a pin dedicated to a PCI Management Event (PME) signal. The driver required for this pin is intended to be open drain and active low. This driver has requirements beyond those required for standard open drain signals that are shared between devices that are both powered on and off. One requirement of the driver of the PCI pin is that a voltage applied to a network of integrated circuit pins will not cause damage to any device associated with a pin, especially when that component is not powered. Another requirement is that the device must not pull the PCI pin low unless each pin of the network is asserted, including a state defamed by xe2x80x9cD3 cold.xe2x80x9d
As implied above, any device implementing a PME function must not be damaged when the device is not powered and a PME signal is provided to the PME pin of the device. Also, the PME signal driver of the device that drives the signal must not present a low impedance path to any voltage source when the device is not powered. These requirements ensure that the PME signal network will continue to properly fiction when a mixture of powered and unpowered devices have their PME pins coupled together.
Most commonly available open drain output drivers, even those designed to be 5V tolerantxe2x80x94the drivers are powered by a lesser voltage, such as 3.3Vxe2x80x94do not meet the above requirements. For example, FIG. 1 illustrates a typical 5V tolerant open drain driver 100 coupled to a pad 130 and a buffer 140. Open drain driver 100 includes series pull-down devices that are illustrated as transistors 110 and 120. The gate of transistor 110 is tied to the power supply that typically provides 3.3V. Thus, transistor 110 is configured to be on or in a conductive state.
While the power supply is providing a voltage to transistor 110, an incoming voltage provided from pad 130 is divided by transistors 110, 120. As a result, the voltage from pad 130 is not applied across only one device. However, if the power supply is not providing a voltage (powered off), then the entire voltage form pad 130 is applied across transistor 110. In particular, if the voltage from pad 130 is 5V and the gate voltage of transistor 110 is about 0V, then there is a drain to gate voltage drop of 5V-0V. If transistor 110 has a gate oxide that cannot tolerate such a voltage drop, the gate oxide will be damaged.
Accordingly, a need exists for an open drain driver that is tolerant of voltages higher than the power supply of the driver and that will not provide a low impedance path to a voltage source when not powered. The present invention meets this need.
The present invention includes a pad driver circuit that has a driver, a power-on circuit coupled to the driver and a power-off circuit coupled to the driver. The power-on circuit is coupled to a pad via the power-off circuit. The power circuit provides a high impedance path between the pad and a power supply, particularly when the power supply is off. The power-off circuit provides a voltage to the driver when the voltage signal is received. The driver is an open drains driver that includes series pull-down devices. The pad driver further includes a second power off circuit coupled to the driver.
The power-on circuit provides a power supply voltage to the driver when a power supply is on. The power-on device preferably includes an inverter coupled to receive a power supply voltage and a clamp coupled to receive an output of the inverter wherein the clamp provides a voltage responsive to the power supply voltage.
In operation, the present invention provides for a pad driver that is tolerant of voltages that are greater than a power supply voltage provided to the pad driver. To that end, the power-off circuit steps down a voltage received from the pad. This stepped down voltage is then provided to the driver. The present invention also provides a high impedance path between the pad and the power supply when the power supply is off and a voltage signal is received from the pad.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings in which details of the invention are fully and completely disclosed as a part of this specification.